The present invention relates generally to fluid seal arrangements for sealing high pressure areas from low pressure areas and in particular to improvements to brush seals.
Brush seals have been developed for sealing high pressure regions from low pressure regions for, in particular, gas turbine engine applications where a seal is required between relatively movable parts, typically between rotor shafts and a stationary housing. The seal prevents or restricts leakage flow along the shaft. Such brush seals offer improved sealing as compared to conventional labyrinth type seals and can better accommodate radial movements of the shaft.
Brush seals comprise a layer of bristles which are sandwiched between annular front and backing plates which are mounted on and extend radially from the stationary housing surrounding the shaft. The backing plate typically extends radially further inwards towards the shaft than the front plate with the bristles extending yet further still with the bristle tips typically wiping against the surface of the shaft. Although the bristles provide a significant degree of sealing it has been found that the clearance between backing plate and the shaft has a significant effect on seal performance with smaller clearances reducing the leakage flow. However since, unlike the bristles, the backing plate is solid and fixed a minimum clearance between the backing plate and the shaft is required in order to accommodate anticipated radial movement of the shaft. Such movement is caused by thermal growth, centrifugal growth, eccentric mounting of the shaft, shaft vibration and transient movement due to loads on the shaft.
One proposal to address this problem and provide an improved seal is described in U.S. Pat. No. 5,351,971. In this proposal a radially movable backing plate is suggested which can move radially when contacted by the shaft in order to accommodate the anticipated radial movement of the shaft. Since the backing plate can now move radially a smaller clearance between the backing plate and shaft can be used as compared to a seal with a fixed backing ring and the seal performance can be improved.
A problem however with this proposal is that the shaft must contact the backing plate in order to move the backing plate. Such contact will generate friction between the backing plate and the shaft even when non abrasive low coefficient coating are used, which will generate heat and will cause wear of the backing plate and/or shaft. Such wear increases the clearance between the backing plate and shaft which reduces the seal performance. Furthermore wear of the shaft will reduce its strength and may give rise to a stress concentration requiring the shaft to be prematurely replaced or in extreme cases could lead to shaft failure.
It is therefore desirable to provide an improved seal arrangement which addresses the above problems of contact between the backing plate and shaft whilst minimising the clearance therebetween to provide improved seal performance and/or which offers improvements generally.
According to the present invention there is provided a brush seal comprising a plurality of bristles packed together in a bristle layer with the bristles mounted on and extending from a first component towards a facing surface of a second component, and a movable plate disposed substantially parallel to the bristles and adjacent to the bristle layer, the plate being movable relative to the bristle layer in a direction parallel to the bristle layer; characterised in that the movable plate and an edge of the movable plate adjacent to and facing the facing surface of the second component are arranged to, in use, air ride on the facing surface of the second component.
Preferably the edge of the movable plate adjacent to the second component is profiled so as to promote air riding of the movable plate adjacent to the facing surface of the second component.
Alternatively the edge of the movable plate adjacent to the second component comprises an enlarged foot member which extends from the remainder of the movable plate so as to define an enlarged surface substantially parallel to and facing the facing surface of the second component.
Furthermore the plate recess walls may define a recess in the edge of the movable plate adjacent to the second component, with the recess defined in the movable plate having an open side facing the facing surface of the second component. A duct within the movable plate may interconnect the recess with a source of pressurised fluid and in operation supplier pressurised fluid to the recess. The depth of the recess may vary along the length of the edge of the movable plate adjacent to the second component.
Preferably the facing surface of the second component in the region facing the movable plate is sufficiently smooth so as to promote air riding of the movable plate.
A support plate may extend from the first component towards the second component, at least a portion of the support plate abutting the movable plate and supporting the movable plate in a position adjacent to the bristles. Preferably a first portion of the support plate is spaced from the movable plate with a second portion of the support plate extending towards and abutting the movable plate such that a chamber is defined between the support plate and the movable plate. A duct may interconnect the chamber with a source of pressurised fluid.
The movable plate is preferably segmented.
At least a portion of the movable plate adjacent to the bristle layer may be spaced from the bristle layer. A chamber may thereby be defined by between the movable plate and the bristle layer. Preferably, in use, pressurised fluid is arranged to be supplied to between a region between the movable plate and the bristle layer.
Preferably the first component is a stationary housing, the second component is a rotatable shaft and the movable plate is annular.
The movable plate may be disposed either downstream or upstream of the bristle layer.